Combining control cables in Substation conduits - Best Practice
Combining control cables in Substation conduits - Best Practice
(OP)
What is typically viewed as best practice for combining control cables in UG conduit in Substations? Lets assume typical control cables consists of CT wiring, PT wiring, 125VDC control, 120VAC power, 48V control.
- Is it typical to combine CT wiring with other 125V DC control wiring?
- Is it typical to combine CT & PT wiring?
- Is it typical to combine 120VAC (or any AC power) with CT wiring or DC control?
- Is it typical to combine different voltage levels for DC voltages (IE 48V and 125VDC)?
I know that the NEC and other codes likely allow for combination of the above (IE 600V class wiring) so was looking for more of a "best practice" and reliability perspective.
Thanks
- Is it typical to combine CT wiring with other 125V DC control wiring?
- Is it typical to combine CT & PT wiring?
- Is it typical to combine 120VAC (or any AC power) with CT wiring or DC control?
- Is it typical to combine different voltage levels for DC voltages (IE 48V and 125VDC)?
I know that the NEC and other codes likely allow for combination of the above (IE 600V class wiring) so was looking for more of a "best practice" and reliability perspective.
Thanks
RE: Combining control cables in Substation conduits - Best Practice
I’ll see your silver lining and raise you two black clouds. - Protection Operations
RE: Combining control cables in Substation conduits - Best Practice
That said I agree from a maintenance focused standpoint it makes sense. Are there specific reasons in your experience that support the maintenance aspect of that approach? Replacement of failed cable, easy identification of wire types in a conduit or leaving a conduit?
RE: Combining control cables in Substation conduits - Best Practice
RE: Combining control cables in Substation conduits - Best Practice
I have not seen strict restrictions on which control cables can run in the same conduit. If possible, SCADA status cables are often somewhat separated. In retrofit applications, there is often very limited conduit space available.
I have not personally seen cables where CT circuits are mixed with any other type of signal. Our typical new circuit breaker installation uses 10 separate cables, which does get to be kind of pain to manage that many separate cables. During initial construction, a smaller number of larger cables would have made cable identification somewhat easier. I think one of my neighboring utilities uses control cables with more than 15+conductors, so I suspect they might be mixing several kinds of signals in one cable.
For control of a transformer load tap changer, I have seen both the raise/lower control and a single phase PT circuit share a single 9 conductor cable. Whether this qualifies as a best practice, I am not sure.
RE: Combining control cables in Substation conduits - Best Practice
An industrial owned substation, on the other hand, will probably get by with what ever the design consultant is doing at the time of the design. They have a tendency to be much more one-off than any utility would typically put up with.
The NEC, which would apply to the industrial substation but not to the utility substation, would allow anything to be grouped together so long as the lowest insulation rating in the cable is higher than the highest voltage used. Quick, what voltage do you pick for a CT secondary? Is 600V insulation sufficient on the secondary of a C800 CT? I don't know and I doubt that there are many AHJs that could give a good answer as to why or why not. I've had to explain to the AHJ why a 15kV circuit breaker can use a 600V insulated CT or why a 600V rated window CT can be used around a 15kV insulated cable.
But I'll be glad when most of this can be put to rest in large substations where a breaker gets a couple of DC power circuits and a few fiber pairs and that's it. Probably many years yet before we do our first one that way, but I think the time will come.
I’ll see your silver lining and raise you two black clouds. - Protection Operations